In some circumstances the vulnerable parameter is exploitable only if the user provides a postfix to be appended to the injection payload. Another scenario where these options come handy presents itself when the user already knows that query syntax and want to detect and exploit the SQL injection by directly providing a injection payload prefix and/or postfix.

As you can see, the injection payload for testing for custom injection is:

id=1%27%29%20AND%207433=7433%20AND%20%28%27test%27=%27test

which URL decoded is:

id=1′) AND 7433=7433 AND (‘test’=’test

and makes the query syntatically correct to the page query:

SELECT * FROM users WHERE id=(‘1’) AND 7433=7433 AND (‘test’=’test’) LIMIT 0, 1

In this simple example, sqlmap could detect the SQL injection and exploit it without need to provide a custom injection payload, but sometimes in the real world application it is necessary to provide it.